The present invention relates to a new and improved method and apparatus for applying a substantially strip-shaped powder layer, particularly on a welding seam of a container or vessel, such as a can body, and also to the can body itself.
It is known to coat the welded longitudinal seams of can bodies with electrostatically charged powder. This method is utilized for can bodies in which sensitive filling materials are stored. Polymeric resins, such as for example epoxy resin, polyethylene and the like are frequently utilized as the powder for coating. The powder finely distributed in an air stream is supplied to the welding seam and electrostatically charged shortly before making contact with the seam.
When the powder leaves the transport passage it is distributed over a region which is wider than the dimension of the welding seam. It also therefore coats such parts of the can body which must not be coated. On the other hand, a quantity of powder applied directly onto the welding seam, which often has a sharp edge, is insufficient to guarantee a reliable coating of the seam.
U.S. Pat. No. 3,713,862, granted Jan. 30, 1973, discloses a method in accordance with which there is provided a band which covers the lateral regions of the seam, so that a definite small and uniform powder strip is applied onto the outer side of the seam. Coating of the inner side of the seam, however, is not possible with this method, since the covering band provided with slots cannot be guided through the welded or, in other words, closed can body. A uniform powder application over the whole cover region is furthermore not suitable. When there is applied a sufficient quantity of powder to reliably cover the welding edges of the seam, too much material lies in the lateral regions. Taking into consideration the high price of the powder, this is of course retained low, the welding seam becomes insufficiently coated.
German Offenlegungsschrift No. 2,933,641, which is cognate to U.S. Pat. No. 4,215,648, granted Aug. 5, 1980, describes a powder applying apparatus in which the lateral powder dissipation is prevented by partial separation of the air from the powder shortly before the discharge of the powder from the spraying head. The powder is separated from the air shortly before it reaches the seam by centrifugal separation, for example with a return track curve, then is chamber, and at that location charged by a row of electrodes also arranged parallel to the seam. The air stream which is supplied parallel to the powder stream also into the spraying chamber positively mixes with the powder and forms a powder-air cloud. This apparatus, however, does not render possible any concentrated application of the powder at the seam with a small quantity of powder particles dissipating into the surroundings. A predetermined distribution of the powder transverse to the seam region is therefore not possible.
In a further development of the above-mentioned German Offenlegungschrift No. 2,933,641, reference being had to European patent application Ser. No. 54,575, the spraying chamber is composed of a porous material. Gas or air is blown into the spraying chamber through the porous material in order to blow the powder particles which are brought substantially parallel to the welding seam, against the seam region. A predetermined application which covers only a very small region of the seam is excluded with this apparatus. This apparatus positively leads to a great powder region.
Swiss Pat. No. 603,249 discloses a spraying head for a powder applying apparatus, which is provided with a plurality of bars and guiding sheets extending transverse to the flow direction of the powder-air mixture. The bars and sheets brake the powder-air stream and deflect it against a discharge slot which is laterally limited by flexible strips. A further deflecting element is provided, formed by a plurality of guiding sheets extending transverse to the slot. With the air of additional air blown through i.e. jets or nozzles, druses, the powder-air stream is additionally supported in the rear region of the spraying head.
The utilization of baffles of different shapes for braking and deflecting the air-powder mixture results in a very complicated construction of the spraying head, which also has a tendency towards clogging, for example powder nesting. The distribution of powder obtained by guiding and braking elements takes place at the cost of a very high throughflow quantity of powder and air, which must be withdrawn in greater part by aspiration and then recycled again. In addition to the cost of pressure air and suction air, the recycling and loss of a part of the returned powder leads to further losses.
All known powder applying arrangements or apparatuses produce directly or indirectly in a spraying chamber a powder-air cloud which is electrostatically charged in the spraying chamber and then applied against the seam which moves over the spraying chamber, due to its charging, as well as due to the superpressure and the air stream in chamber. Despite efficient suction hoods which are located above the spraying chamber, many powder particles still will be deposited on the outer side of the can body. The greatest mass of the powder particles supplied to the spraying chamber travel into the aspirating device and must be cleaned in a recycling device.